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The Science of Roof Failure

hurricane winds, hurricane prepardeness, spray polyurethane waterproofing

Wind produces inward pressure on walls and the roof facing the wind and outward negative pressure on the opposite walls and roof. At the corners wind pressure can be twice as high! As the storm hits the windward side of the roof, positive pressure starts pushing on the roof loosening the tiles beginning the water intrusion process.

 

The positive wind pressure on windward side of the roof passes over the back or the roof. This creates very strong amounts of negative pressure pulling upwards on the back of the roof. hurricane winds, hurricane prepardeness, spray polyurethane waterproofingThis uplift pressure can often be the most destructive force the roof will face. A roof is designed with the nails going straight down through the sheathing into the truss. The suction of the negative pressure causes large amounts of uplift pressure to be place on the sheathing pulling the nails in a vertical direction back out of the wood, similar to prying a nail out with a hammer. The vertical direction of this pressure is the easiest way to remove the nails holding the roof together.

 

hurricane winds, hurricane prepardeness, spray polyurethane waterproofingBuilding codes have been improved over recent years due to the high percentages of roof failures during minor hurricanes. These improvements require nailing patterns to be closer together and larger nails to be used. The goal of these improvements is to provide sufficient uplift resistance for pressures associated with a category III hurricane. However if the building envelop is breached, internal pressures can double causing roofs to fail in even minor hurricanes. Category IV or V hurricanes can produce sufficient uplift pressure to separating the sheathing from the truss without a window or door failure.
 

During a hurricane the load placed on the roof must be transferred from the top of the roof through the sheathing, into the rafters and down through the wall. Do to the design of roofs; the weakest link in this load path is at the connection of the sheathing to the rafters by a nail every 12 inches.  FOAMSEAL hurricane adhesive provides a continuous bond on both sides of the truss with the sheathing greatly increasing the strength of the connection while creating a water-tight seal throughout the roof.  

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Updated:  09/19/2013

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